In wavelength modulation spectroscopy (WMS) for measuring the concentration of a gas component in a gas sample a portion of the light of a tunable light source, usually a continuously tunable laser such as a diode laser, is passed through a reference gas comprising the known gas component or another suitable gas component of constant concentration. Afterwards the light is detected by a reference detector. Another portion of the light is directed to a monitor detector for normalization purposes. Yet another portion of the light is passed through the gas sample and thereafter to a measuring detector. The light emitted by the light source is modulated with a frequency fm, while the wavelength is swept over a molecular absorption line of the gas component. As the light propagates through the reference gas or gas sample, respectively, wavelength dependent absorption converts some of the wavelength modulation into an intensity modulation of the light. Thus, the light will have an overtone spectrum generated by the absorption process, the harmonic content of the spectrum being dependent on the width and shape of the molecular absorption line in the gas and the etalons in the spectroscopy system. When the light then impinges onto the reference detector or measuring detector, respectively, the detector outputs contain AC components at the modulation frequency fm and its higher harmonics Nfm (N=2, 3, 4, etc.). Demodulating the respective detector outputs at one of said higher harmonics Nfm shifts the measurement from frequencies near DC, where the light source is noisy, into a higher frequency range, where the noise is lower, thus improving the measurement sensitivity.
The modulation of the emitted light can most conveniently be accomplished by modulation of the injection current of the diode laser, which imposes modulation on the wavelength and to some extend on the intensity of the emitted light. As the demodulated Nfm absorption signal depends not only on the concentration of the measured gas but also on the modulation parameters of the light source, variations of these modulation parameters can affect the accuracy of the measurement.